Patients with Cystic Fibrosis (CF) develop chronic airway infections with the opportunistic gram negative bacteria Pseudomonas aeruginosa. Airway inflammation and neutrophilic infiltration without bacterial destruction characterize these infections. It has been shown that Pseudomonas isolated from Cystic Fibrosis patients have specific, virulence-associated modifications in their lipid A structure. These modifications, which include substitutions with palmitate and 4-aminoarabinose, are responsible for resistance to cationic antimicrobial peptides (CAMPs), an important component of innate immunity and Polymyxin, a CAMP antibiotic. The enzymes responsible for the biosynthesis of 4- aminoarabinose-lipid A are clustered in an operon recently renamed to ArnBCADTEF. Mutation of any of these genes abolishes 4-aminoarabinose addition to lipid A and resistance to CAMPs. Based on our previous structural and biochemical work with ArnA we have developed a strategy for selective inhibition ArnA . In this proposal we also target ArnD, a hypothetical deformylase essential for the biosynthesis of 4- aminoarabinose-lipid A, for biochemical and structural characterization. Inhibition of any of these enzymes would abolish Pseudomonas aeruginosa resistance to antimicrobial peptides, therefore greatly enhancing the host immune response against chronic infections with Pseudomonas. To improve both the quality of life and the survival age of CF patients it is crucial that new strategies are developed to manage their pulmonary infections. Given the chronic nature of these infections preventing or abolishing resistance is a fundamental problem. This proposal focuses on the characterization of bacterial targets that mediate resistance to CAMPs.